Today's transmission networks, especially digital ones, are growing increasingly complex. Any interruption, however brief, of the transmission process can prove extremely costly. It is, therefore, highly desirable that test means be provided to determine as quickly and thoroughly as possible the manner in which individual transmitting and receiving stations in such a network are functioning.
To this end, transmission networks usually include a central device which remotely controls test operations. A well-known test, called loopback or wrap test, consists in looping a test signal from the central control device through the station to be tested and back to the central device for measurement in order to determine alterations due to the transmission process and, consequently, the existence of faults in that part of the network. To exchange the effectiveness of this test, the test signal, prior to being sent back to the central control device, should preferably travel through the greatest possible part of the station being tested; that is, the test signal should reach a point as close as possible to the transmission line that is to be driven, through an analog circuit called interface circuit, by the transmitter in said station. Thus, the interface circuit should be designed to allow the test signal to pass therethrough without driving the attached transmission line in the process. A simple solution would be to use electromechanical relays to isolate the interface circuit from the transmission line while the test signal is being fed back to the central control device. Unfortunately, electromechanical devices are both unreliable and expensive.